Exploring the Parameters Controlling Product Selectivity in Electrochemical CO2 Reduction in Competition with Hydrogen Evolution Employing Manganese Bipyridine Complexes

Selective reduction of CO2 is an efficient solution for producing nonfossil-based chemical feedstocks and simultaneously alleviating the increasing atmospheric concentration of this greenhouse gas. With this aim, molecular electrocatalysts are being extensively studied, although selectivity remains an issue. In this work, a combined experimental–computational study explores how the molecular structure of Mn-based complexes determines the dominant product in the reduction of CO2 to HCOOH, CO, and H2. In contrast to previous Mn(bpy-R)(CO)3Br catalysts containing alkyl amines in the vicinity of the Br ligand, here, we report that bpy-based macrocycles locking these amines at the side opposite to the Br ligand change the product selectivity from HCOOH to H2. Ab initio molecular dynamics simulations of the active species showed that free rotation of the Mn(CO)3 moiety allows for the approach of the protonated amine to the reactive center yielding a Mn-hydride intermediate, which is the key in the formation of H2 and HCOOH. Additional studies with DFT methods showed that the macrocyclic moiety hinders the insertion of CO2 to the metal hydride favoring the formation of H2 over HCOOH. Further, our results suggest that the minor CO product observed experimentally is formed when CO2 adds to Mn on the side opposite to the amine ligand before protonation. These results show how product selectivity can be modulated by ligand design in Mn-based catalysts, providing atomistic details that can be leveraged in the development of a fully selective system.


■ INTRODUCTION
Catalysis is crucial for modern society and life in general. Catalytic processes occur in the mitochondria, where glucose is converted into energy, in refineries, where crude oil is converted into fuels, in the polymer industry, facilitating the production of plastics for computers, houses, and clothes, and in the pharmaceutical industry, where vital medicine is produced. In fact, 85% of all products manufactured have been produced with the assistance of a catalyst, and in 90% of all chemical processes, at least one catalyst is employed. 1 Despite the huge number of existing catalysts, it is still of high priority to develop new and more efficient ones that with low cost, low energy consumption, and low environmental impact selectively convert building blocks into target compounds. 2 Electrocatalysts show great promise in this respect, as they can be powered by renewable energy sources while operating at ambient temperature and pressure. 3 For instance, electrocatalysts are widely used in the electrolysis of water, where water is decomposed into oxygen and hydrogen gas, of which the latter is a promising carbonneutral fuel that does not emit harmful exhaust gases. 4,5 Furthermore, research regarding electrochemical CO 2 reduction is advancing with the prospect of decreasing CO 2 pollution while converting the greenhouse gas into value-added fuels and chemicals. 6−8 A recurring challenge within catalysis is to obtain a high selectivity toward a single product. Within the family of metalbased molecular catalysts, a lot of research has focused on secondary coordination sphere effects and how different functional groups influence the selectivity of the complexes. 12−16 Recently, we showed how the selectivity of CO 2 reduction by Mn bipyridine (bpy) catalysts (1a*−c*, Figure 1) changes from CO, when hydroxyl (1b*) or alkyl groups (1c*) are close to the metal center, to HCOOH, when amines (1a*) are placed in the vicinity of the metal. 9 As presented in Scheme 1, the amines function as proton shuttles, transferring protons from the ligand to the metal center, thereby facilitating the formation of a Mn hydride, 4, which is the key intermediate in the HCOO − /HCOOH and H 2 pathways. Additionally, the presence of the amine functionalities is also known to stabilize the carboxylate intermediate, 7, by forming an intramolecular hydrogen bond, which then favors CO formation (blue pathway, Scheme 1). 17−19 Since amines accelerate all three pathways, the selectivity originates from other contributing factors, such as the nature of the metal, 20 geometrical structure, 21 hydricity, 22 and pK a . 23 In the case of Mn complexes, CO has been the dominant product for the nonamine-containing complexes (1b* and 1c*).
Since the abovementioned pathways always compete with each other, especially when there is a bifurcation between the H 2 and HCOO − pathways from a common Mn hydride intermediate, we explored how the structure affects the selectivity of the catalysts within the Mn bpy family. The structure has already been shown to affect the catalytic activity, as an unsubstituted Mn bpy complex forms less reactive dimers upon reduction, 24 which is avoided by introducing bulky substituents on the ligands. 25 However, the fundamental understanding of how the molecular structure and product distribution are linked is still missing. To investigate this correlation more carefully, three different Mn tricarbonyl complexes with macrocyclic bipyridine ligands were synthesized (1a−c, Figure 1). Using our previously reported complex 1a*, bearing tertiary amines in the secondary coordination sphere, as a reference catalyst, here, we aim to explore the effect of locking all the amine groups in a relatively enclosed structure by introducing a macrocyclic linker. Furthermore, we examine if the central heteroatom of the linker (N or O, for complexes 1a and 1b, respectively) affects the product distribution, while the size of the macrocyclic ring is evaluated with the one-atom shorter complex 1c.
With these complexes in hand, the different possible pathways proposed in Scheme 1 are scrutinized to get a clearer The arc connects the amine-bearing pendants in the secondary coordination sphere, while only one of the sides is included for simplicity. LO and HO stand for low overpotential and high overpotential, respectively. 9−11 mechanistic understanding of the factors affecting the product distribution. Specifically, the order of competing protonation and CO 2 insertion reactions is considered, and the energy barriers of these steps are evaluated by density functional theory (DFT) calculations. Furthermore, information about the flexibility of the molecular structures is gained through ab initio molecular dynamics (AIMD). This fundamental study builds on an electrochemical assessment, taking complex 1a as a starting point, in which the key intermediates formed during reduction are identified using infrared spectroelectrochemistry (IR-SEC). Finally, based on the mechanistic insight obtained from our combined experimental and computational studies, we propose a general guide for rational ligand design.
■ RESULTS AND DISCUSSION Synthesis and Characterization of Complexes. The three macrocyclic complexes (1a−c) were synthesized as exemplified for 1a in Scheme 2. The synthesis was envisioned to proceed through a double reductive amination from dialdehyde A using a small excess of the corresponding di-or triamines. To minimize the formation of larger ring sizes or polymers, the reaction was performed using a low concentration of dialdehyde A (55 mM) in tetrahydrofuran (THF), and a syringe pump was employed to slowly add the amines over five hours (Scheme 2a). After stirring for 48 h, full conversion of dialdehyde A was obtained in all three cases, and the macrocyclic ligands could be obtained in 13−48% yield after purification. Complexes 1a−c were obtained by metalation of the respective ligands using Mn(CO) 5 Br in THF at 55°C yielding the complexes as yellow solids in good yields (Scheme 2b). The complexes were analyzed by 1 H NMR, 13 C NMR, ATR-IR, and high-resolution mass spectrometry (HR-MS). Furthermore, the 1 H NMR and 13 C NMR chemical shifts of complex 1a were calculated and found to be in good agreement with the experimental values (Table S1). Figure 2 shows the single-crystal X-ray diffraction structure of 1a, and its crystallographic data are listed in the Supporting Information (Section 2). In the solid state, the Mn center adopts a facial octahedral geometry, having the modified bpy ligand and two carbonyl ligands in the equatorial plane, while the bromide is placed in the axial position together with the third carbonyl group pointing in opposite directions. Interestingly, the Mn−Br bond is in the direction opposite to the aliphatic amine ligand. The bpy plane is slightly distorted (N−C−C−N dihedral angle, φ = 11.6°), while the adjacent phenyl rings are rotated out of the plane 60−80°. The linker between the two phenyl rings is situated below the plane of the bpy ring system. This is an interesting observation as it implies that no amines are in close vicinity to the Mn center, which we previously have shown is important for the product selectivity. 9 The single-crystal X-ray diffraction structures of 1b and 1c are shown in Figure 2, and crystallographic data are listed in the Supporting Information (Section 2). The coordination sphere of Mn is facial octahedral for both complexes, and the general observations mentioned above for 1a also hold true for 1b and 1c. Only one isomer with the Br in the opposite direction to the ligand (exo) is observed for 1a−c in solution, which is also supported by DFT calculations (TPSSh-D3/def2SVP//TPSSh-D3/def2TZVP, def2TZVPD, see Section 3 in the Supporting Information for details). It was found that the exo isomers of 1a, 1b, and 1c were 8.4, 8.2, and 7.4 kcal mol −1 , respectively, more stable than the corresponding endo isomers, characterized by Br and amine pointing in the same direction (Scheme S1).
AIMD Simulations. Formation of the hydride intermediate in the H 2 pathway (Scheme 1) requires the proximity of the chelating-ligand amines to the metal center, which seems to be excluded by the crystal structure of 1a ( Figure 2). However, this reaction is preceded by the two-electron reduction of 1a and the concomitant elimination of bromide, yielding an anion species 2a − , and the protonation of the amine moiety of the latter to form 3a (Scheme 1 and Figure 3). Hence, we studied the fluxional behavior of the ligand in these two species by means of DFT AIMD (PBE-D3/DZVP) as implemented in the CP2K program 26−28 followed by static DFT calculations. Figure 3 shows the time evolution (within a production trajectory of 25 ps) of the Mn···H distances for complex 2a − , involving the three amine N atoms of the ligand: the middle one (N M ) and the two on the sides (N S1 and N S2 ). The side N atoms were initially considered different because the ligand is asymmetric in the crystal structure. However, their dynamic behavior should be equivalent at longer trajectories.
The AIMD simulation of 2a − reflects the dynamic structure of the chelating ligand, in which the variation of the Mn···N distances has wide amplitudes [1.  corresponds to the most flexible amine, is located at the position furthest from the metal center. Interestingly, this behavior is reversed when 2a − is protonated to 3a ( Figure 3); i.e., the shortest average Mn···N(H) distance is observed when the proton binds to the middle N M atom, yielding an average value of 3.39 Å (measured after the sudden change at ∼2 ps). For all three protonated isomers, 3a-N M , 3a-N S1 , and 3a-N S2 , the time evolution of the Mn···N(H) distances is strongly correlated to that of the corresponding Mn···H(N).
Therefore, the shortest average Mn···H(N) distance was also observed for N M , with a small value of 2.38 Å, suggesting the presence of a hydrogen bond between Mn and the H−N moiety. This is consistent with the dramatic shortening of the average Mn···N M distance by 3.35 Å upon protonating 2a − , and the narrowing of its amplitude to 0.78 Å. In contrast, the Mn··· N S (H) and Mn···H(N S ) distances in 3a-N S1/2 fluctuate, as indicated by their large amplitudes in the range of 2.31 and 3.09 Å. This indicates that even though the formation of a hydrogen bond is feasible (shortest Mn···H(N S1 ) distance = 2.30 Å), this interaction is weaker than that in 3a-N M (shortest Mn···H(N M ) distance = 1.87 Å) ( Table S2).
The fluxional behavior of 2a − and 3a-N X (X = M, S1, and S2) can be ascribed to the rotation of the Mn(CO) 3 core ( Figure  S1). The AIMD simulation of 2a − shows that this rotation causes an oscillation of the metal axial vacancy in between two positions: one pointing to the triamine bridge (endo) and the other to the opposite direction (exo). In the endo form, the interaction between the N atoms and the vacancy is repulsive because both moieties have lone pairs ( Figure S2). Conversely, in the 3a-N M complex, this interaction becomes attractive, which yields a Mn···H−N hydrogen bond. The exo to endo flip can be observed during the initial 5 ps of the AIMD of 3a-N M ( Figure  3), in which both the Mn···N M (H) and Mn···H(N M ) distances undergo a sudden shortening of ∼3 Å.
The calculations thus show that, despite the long distance observed in the crystal structure of 1a, the middle amine can approach the metal center in the crucial intermediate 3a, facilitating the formation of the hydride complex 4a. The less flexible side positions could also allow the formation of a hydride, but it seems less preferred.
Electrochemical and Spectroscopic Evaluation. Electrochemical experiments were conducted to determine how the macrocyclic bipyridine ligands influence the electrocatalytic properties of the manganese complexes. Cyclic voltammograms of complexes 1a−c are recorded in 0.1 M Bu 4 NBF 4 /MeCN solutions under first Ar-and then CO 2 -saturated conditions (Figure 4 and Figures S3 and S4). Analogously to our findings for complex 1a*−c*, 9 all three complexes present one reduction wave at around −1.69 V vs Fc + /Fc under Ar atmosphere. According to the previous mechanistic interpretation, 9,25 this reduction wave is the result of a consecutive electron transfer− chemical reaction−electron transfer (ECE) mechanism where the first one-electron reduction of Mn(bpy-R)(CO) 3 Br (complex 1 in Scheme 1) is followed by the dissociation of Br − , generating a neutral intermediate 2. 11 The second electron is transferred from the electrode to 2 instantly since 2 is formed very close to the electrode surface due to the rapid dissociation of Br − and is easier to reduce than 1 itself. Thus, the overall twoelectron reduction gives rise to the anionic state 2 − , 29 as shown in Scheme 1. The process is diffusion-controlled with unaltered electron stoichiometry according to the Randles−Sěvcǐ́k equation ( Figure S5). 25 The above interpretation was also confirmed by DFT calculations (see the Supporting Information for further details) taking 1a as an example (Scheme 3). The first reduction from 1a to 1a − has been calculated to take place at E 1 = −1.81 V vs Fc + / Fc, after which Br − is easily expelled to form 2a with ΔG = −7.1 kcal mol −1 . The analysis of the spin density in 1a − (Table S3) shows that the electron is localized at the bpy ligand as found in similar complexes. 30   at E 2 = −1.59 V vs Fc + /Fc. Taken together, these three processes yield a two-electron reduction taking place at E 3 = −1.60 V vs Fc + /Fc. This overall potential E 3 is positively shifted compared to E 1 due to the follow-up dissociation of Br − , which is in good accordance with the experimental potential of −1.69 V vs Fc + / Fc.
Infrared spectroelectrochemistry analysis of 1a, in the absence of CO 2 and a proton source, further uncovers the formation of anionic species 2a − since the CO stretches shift from 2021, 1936, and 1906 cm −1 (1a) to 1909 and 1807 cm −1 during the voltammetric sweeping ( Figure S6), in alignment with previous reports. 25,33,34 In addition, the stretch at 1982 cm −1 and the bump at 1884 cm −1 stem from manganese hydride 4a (vide infra in Schemes 4 and 5). 9,35 Its appearance is a consequence of the amine-bearing ligand, which shuttles protons from residual water in the electrolyte to the metal center to generate 4a. Hence, we assign the faint peak at around −2.10 V vs Fc + /Fc in Figure 4 under Ar to the reduction of 4a. 36,37 A similar mechanism was outlined by us for complex 1a*. 9 Now addressing the CO 2 -saturated electrolyte ([CO 2 ] ≈ 0.28 M), 38 significant changes occur to the cyclic voltammetric response. Notably, the oxidation waves of the anionic species disappear (see Figure 4 and Figures S3 and S4). A prepeak appears at −1.55 V vs Fc + /Fc for 1a and 1c, which is associated with the reduction of the solvent-coordinated cation originating from partial solvolysis of 1a and 1c. 24 This behavior was also reported for analogous Mn complexes. 35,37 In addition, DFT calculations are in agreement with this assignment. We found that the solvent-coordinated complex, 2a + -MeCN, is also generated through a two-electron oxidation process of 2a − at E 4 = −1.22 V vs Fc + /Fc followed by the exergonic coordination of MeCN with ΔG = − 6.4 kcal mol −1 , which is reduced at a potential of E 5 = −1.36 V vs Fc + /Fc (Scheme 3). At the same time, a dramatic current enhancement is detected at −2.10 V vs Fc + /Fc. This is in accordance with a high-overpotential pathway, in which the hydride species 4 is reduced and subsequently enters the catalytic cycle. 9 The trace crossing in Figure 4 and Figures S3 and S4 indicates an acceleration of the catalytic behavior on the reverse scan (see Section 5 in the Supporting Information for a detailed explanation). 39 The specific catalytic effect of residual water (∼0.034 M) in combination with CO 2 (Figure 4) is discussed thoroughly in Section 6 of the Supporting Information. Upon sequential addition of either 2,2,2trifluoroethanol (TFE, pK a = 35.4 in MeCN, 40 Figure S7) or 2-propanol (iPrOH, pK a ≈ 42 in MeCN, 37 Figure S8), the catalytic current continues increasing, until it levels off or even drops once very high concentrations of TFE (2.0 M) or iPrOH (1.0 M) are employed. Thus, under these conditions, the CO 2 Scheme 3. Computational Study on the Reduction Path of 1a Comprising Two Kinds of Amine Moieties in the Ligand a a All potential values are calculated relative to Fc + /Fc (see the Supporting Information for details). Protonation steps assume that the proton source is CF 3 CH 2 OH/CO 2 .

ACS Catalysis pubs.acs.org/acscatalysis
Research Article reduction reaction is independent of the proton concentration but limited by the regeneration of the catalyst, as often observed for catalytic reactions. 9,25 Upon introducing CO 2 to the IR-SEC experiments of 1a, a mixture of 2a − (1909 and 1807 cm −1 ) and 4a (1982 and 1884 cm −1 ) is observed during reduction ( Figure S9), where the significant increase in the amount of 4a formed can be attributed to the acidification of the solution induced by CO 2 , i.e., CO 2 increases the proton-donating ability of residual water. Introducing a proton source (TFE or iPrOH) makes the signals assigned to 2a − vanish. In contrast, the signals from 4a are strong, highlighting how proton sources facilitate the hydride generation ( Figures S10 and S11). As depicted in Scheme 1, 4 is a vital intermediate in the catalytic cycle, where it either is further protonated to release H 2 or combines with CO 2 to generate a formato complex 5. 22,41,42 A first assessment of the possible CO 2 reduction products can be gained from the IR-SEC spectra recorded in the region of 1750−1500 cm −1 with either TFE or iPrOH as a proton source. The bands at 1691 ( Figure S12) and 1660 cm −1 ( Figure S13) are assigned to the C�O stretches of trifluoroethyl-and isopropyl carbonates, respectively, while the signal at 1609 cm −1 in both cases is attributed to the formation of HCOO − , which is produced when CO 2 inserts into the Mn−H bond. 9,37,43 In addition, the signal at 1609 cm −1 shows a weaker intensity when TFE is present compared with that in the case of iPrOH, which could be related to the diminished generation of HCOO − .
To gain further insights, we turned to controlled potential electrolysis (CPE) to analyze the product distribution on a longer time scale than cyclic voltammetry and IR-SEC experiments. CPE was performed for 1 h at −2.25 V vs Fc + / Fc (∼150 mV more negative than the reduction of the hydride species) for 1a−c using either 2.0 M TFE or 1.0 M iPrOH as a proton source. All experiments were carried out in a twochamber H-cell following the same procedure as reported in our previous work. 9,20 While the reference complex, 1a*, produces HCOOH (or HCOO − ) as the dominant product, 9 complexes 1a−c show high selectivity for H 2 ( Figure 5 and Table S5). In general, only small amounts of HCOO − and CO are produced for all three complexes when TFE is employed as a proton donor; the amounts of HCOO − increase slightly when using the weaker proton donor, iPrOH. This observation can be explained by the competing pathways described in Scheme 1, where 4 either reacts with CO 2 to form HCOO − or H + to produce H 2 , of which the latter option is favored when stronger proton donors are used. 14,35,37,44 This result also suggests that the highest

ACS Catalysis pubs.acs.org/acscatalysis
Research Article energy barriers for the formation of these two products are very similar (vide infra). Specifically, complexes 1a and 1b with the same linker length but different central hetero atoms (N and O, respectively) show both a high Faradaic efficiency for H 2 (FE H2 ) of ∼66% with 2.0 M TFE and ∼48% with 1.0 M iPrOH. The FE HCOO-also displays a decent value of ∼29% for 1a and 1b with 1.0 M iPrOH. These results indicate that, even in the absence of the middle N, the side N in 1b can act as proton shuttle favoring the formation of the intermediate 4b. This is also consistent with the AIMD simulations using 3a-N S1/2 (Figure 3). Shortening the macrocyclic ring by leaving the central heteroatom out (1c) leads to an enhancement of FE H2 , which reaches a maximum of 73% with 2.0 M TFE. At the same time, FE HCOO− is suppressed regardless of the proton source. CO is also produced in negligible quantities (FE CO = 2−6%) in all cases.
In a control experiment using no catalyst, we noted that only ∼2.9 μmol of H 2 was produced in the presence of 2.0 M TFE during the 1 h CPE at -2.25 V vs Fc + /Fc. This is significantly less than the yield of H 2 generated in the presence of 1a (Table S6), thus ruling out the possibility that background reactions contribute to the high FE H2 . As mentioned earlier, 4 can be detected by IR-SEC both for complex 1a* with an open structure 9 and 1a with a closed macrocyclic structure ( Figures  S10 and S11), making the HCOO − /H 2 -generating pathways viable. To account for the change in product selectivity with 1a* and 1a−c, the mechanism for the formation of H 2 , CO, and HCOO − was studied by DFT calculations.
DFT Mechanistic Studies. The mechanism of CO 2 reduction starts with the anionic intermediate 2a − , whose lowest energy isomer was selected from AIMD simulations and optimized using static DFT calculations in solvent (Scheme S2). Three scenarios were considered for the formation of the three reaction products (H 2 , CO, and HCOO − ): two starting with the protonation of the amines (N M and N S ), which favors the formation of a Mn hydride (4a) in an endo conformation (see Scheme 4 and Schemes S3 and S4), and one starting with the direct CO 2 addition or protonation of 2a − in an exo conformation (with CO 2 , or H, and the amines in opposite sides, see Scheme S5).
Scheme 4 outlines the reaction energy profiles for the formation of H 2 , CO, and HCOO − considering the protonation of the middle N as the first step, which is the one that provides the lowest energy barrier for the formation of H 2 . This profile has been poised at a calculated potential of −1.60 V vs Fc + /Fc, which is the potential required to reduce 1a to 2a − , as shown in Scheme 3. As shown previously by us, 9 the amine is protonated by TFE with the assistance of CO 2 yielding 3a-N M and CF 3 CH 2 OCOO − , through an energy barrier of 8.8 kcal mol −1 . From here, proton transfer to the metal center to form 4a is preferred over direct CO 2 addition to the Mn center to form 7a-N M by 1.2 kcal mol −1 . Furthermore, CO 2 addition appears to be endergonic and reversible, whereas the proton transfer is exergonic and irreversible. Once the reactive 4a is generated, there are two competing pathways: (i) CO 2 insertion into the Mn−H bond leading to the Mn−OCHO formato complex (5a) for final HCOO − generation (HCOO − pathway in red) 8,42 and (ii) protonation of the amine moiety facilitating H 2 production (H 2 pathway in green). 22,45 The energy barrier of CO 2 insertion (TS-4) to form 10a is 19.2 kcal mol −1 ; 10a, in which the formate is bonded to Mn by the hydrogen atom (Mn−HCO 2 ) is 18.2 kcal mol −1 less stable than the one bonded via the oxygen atom (5a). Initially, one TS connecting 10a and 5a was found at 8.8 kcal mol −1 , with Mn···H and Mn···O distances of 2.49 and 3.27 Å, respectively. However, upon considering the possibility of connecting 5a and 2a + , we found TS-7, which also connects 10a and 5a but with an energy of 2.6 kcal mol −1 , and longer Mn···H and Mn···O distances (3.14 and 4.04 Å, respectively). An alternative mechanism for the direct formation of 5a from 3a-N M , as it has been suggested for enzymes, 46−48 was also considered without success.

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Research Article mol −1 , shows that the barrier for protonation of the amine to form 6a + -N M is lower than that of CO 2 insertion. Liberation of H 2 , however, requires the proton transfer from the amine to the hydride, which has an energy barrier of 15.1 kcal mol −1 (TS-8).
Overall, the formation of H 2 and HCOO − has similar barriers, as shown by the energies of TS-8 (6.2 kcal mol −1 ) and TS-4 (7.7 kcal mol −1 ), respectively, with generation of H 2 being slightly preferred. This result is consistent with the experimental findings that H 2 formation is preferred over HCOO − (FE H2 = 66% vs FE HCOO− = 29%). On the other hand, the formation of CO (blue pathway) continues by proton transfer (TS-6) from the Mn−CO 2 intermediate (7a-N M ) to form a hydroxyl− carbonyl complex, 8a, which is a well-known intermediate in CO 2 to CO reduction processes. 10,49 Subsequent protonation of 8a (1.1 kcal mol −1 ) followed by proton transfer, forming a tetracarbonyl adduct and final liberation of CO and H 2 O (4.0 kcal mol −1 ), has a barrier of 10 kcal mol −1 but is endergonic by ∼3 kcal mol −1 . The CO pathway has consistently higher energies when compared to the H 2 and HCOO − pathways. Another interesting observation from Scheme 4 is that the H 2 pathway is reversible relative to 4a. Since the reaction takes place under electrochemical conditions, 4a can be reduced to 4a − , with a calculated redox potential of −1.87 V vs Fc + /Fc (Scheme 4), which is consistent with the appearance of the high current density increment in the cyclic voltammograms under CO 2 (−2.10 V vs Fc + /Fc, Figure 4), where 4a − is generated as the bifurcation between the H 2 and HCOO − competing pathways. Hence, the pathways for HCOO − and H 2 formation were also calculated from this reduced intermediate, 4a − (Scheme 5). It is seen that the energy barriers relative to 4a − for H 2 formation (TS-15 = 10.4 kcal mol −1 ) and CO 2 insertion (TS-13 = 14.7 kcal mol −1 ) are lowered by 7.3 and 4.5 kcal mol −1 , respectively, when compared to Scheme 4, where these barriers are 17.7 (TS-8) and 19.2 kcal mol −1 (TS-4) relative to 4a. Consistently, the barrier for H 2 formation is lowered more than that for HCOO − generation, leading H 2 to be the dominant product. Additionally, it was seen that product formation in Scheme 5 was exergonic and irreversible, pointing to a reaction that takes place via 4a − . This finding is in line with the high overpotential pathway reported for complex 1a*. 9 The results shown in Scheme 4 were compared with those considering the protonation of the side amine (Schemes S3 and S4). The main differences observed are as follows: (i) Although the protonation of the side amine is thermodynamically favored over the middle amine (ΔG = −2.5 and 1.0 kcal mol −1 , respectively), proton transfer to the Mn center is preferably assisted by the middle amine, with a lower energy barrier (ΔG ‡ = 21.2 kcal mol −1 vs ΔG ‡ = 13.6 kcal mol −1 )�this finding is consistent with the AIMD simulations, which showed that the middle proton gets closer to the Mn center than the protons on the sides; (ii) the barrier for CO 2 coordination (CO pathway, TS-18) is lower than that for the amine-to-Mn proton transfer (H 2 pathway, TS-17) when the side amine is protonated, whereas the opposite holds true for the middle amine; (iii) after the formation of 4a, both the middle and side amines have similar barriers for the H 2 and HCOO − pathways, with a slight preference for the former.
Finally, we compared the energy profiles shown in Scheme 4 with those obtained with the exo isomer formed by the rotation of the Mn(CO) 3 core in the AIMD calculations (Scheme S5). Compared with the endo isomer, where the formation of 4a is assisted by the N M amine, the exo isomer must go through a direct protonation of the metal to form 4a ex with an energy barrier of 17.2 kcal mol −1 , which is 9.7 kcal mol −1 lower than the direct protonation of the metal in the endo isomer (Scheme S9). Conversely, for the CO pathway, a direct addition of CO 2 to Mn takes place in the exo isomer with an energy barrier of 14.9 kcal mol −1 (TS-27, Scheme S5), which is favored over the direct addition of CO 2 to Mn in the endo isomer by 3.2 kcal mol −1 (TS-33, Scheme S6). As a result, while the formation of 4a (ΔG ‡ = 14.6 kcal mol −1 ) is favored over direct CO 2 addition (ΔG ‡ = 18.1 kcal mol −1 ) in the endo case, the opposite happens in the exo profile, with a preference for CO 2 addition (ΔG ‡ = 14.9 kcal mol −1 ) over hydride formation (ΔG ‡ = 17.2 kcal mol −1 ). These results suggest that the cyclic ligand, once protonated, forms a cagelike structure that protects the Mn center from CO 2 addition, hindering the formation of CO. Despite this, a minor concentration of CO is formed due to the addition of CO 2 in the side opposite to the ligand before protonation (exo isomer). This mechanistic picture is in line with the results obtained experimentally in the CPE measurements.
The preference for H 2 formation over HCOO − can be ascribed to the same effect. In 4a, the cagelike structure of the ligand hinders the addition of CO 2 . Indeed, the opposite selectivity is observed with the open system 1a*. To corroborate this hypothesis, we compared the energy barriers obtained for the protonation of hydride 4a and CO 2 addition for 1a and 1a* (Scheme S7). These calculations showed that both processes have lower energy barriers in the open system 1a*. The protonation of 4a* has an energy barrier that is 3.9 kcal mol −1 lower than that of complex 4a, while the CO 2 addition barrier is significantly lower (by 14.5 kcal mol −1 ), consistent with a change in selectivity from mainly HCOO − for 1a* to mainly H 2 for 1a ( Figure 5). This result was further supported by controlled potential electrolysis of 1a* and 1a in the presence of 2.0 M TFE under Ar atmosphere (i.e., without CO 2 ), which yielded H 2 as the only product. Similar results were obtained for 1b and 1c as electrocatalysts under these experimental conditions (Table S7). Hence, the less efficient production of HCOO − by 1a−c is, most likely, attributed to the low accessibility of CO 2 . The macrocyclic ligand around the metal center induces steric hindrance for the insertion of CO 2 into the metal hydride. At the same time, protons can still be delivered to it by the aminebearing ligands, explaining the preference for H 2 generation over CO 2 reduction. Along this line, it would be expected that the hindrance associated with CO 2 insertion to hydride will be incremented for 1c when a more rigid ring is formed by removing the middle nitrogen atom. Indeed, this is reflected experimentally in the lowest FE for HCOO − (3%) obtained for 1c among 1a−c.

■ CONCLUSIONS
Rational design of molecular complexes is crucial for the development of highly selective and efficient catalysts, but the factors affecting these parameters are not uncovered completely yet. This work addressed how the geometric structure of Mnbased complexes affects the competing protonation and CO 2 insertion steps in the electrocatalytic CO 2 reduction reaction. Depending on the sequence of these steps, different catalytic intermediates are formed, leading to the formation of CO, when CO 2 reacts directly with the metal center, or HCOO − /H 2 if a metal hydride is formed during the first step.
To investigate how spatial geometry around the catalytic site affects the formation of these products, three different Mn(bpy-R)(CO) 3 Br complexes (1a−c, Figure 1) were designed and synthesized. Via a combined theoretical and experimental study, we found several parameters that influence the selectivity of the catalysts. First, ab initio molecular dynamics simulations revealed that the three carbonyl groups rotate freely around the metal center after a 2e − reduction of the complex, leaving the molecular structure more flexible than first anticipated. The outcome of the CO 2 reduction reaction is thus dependent on the stability of the two major conformers, where the catalytic site points either in the same (endo) or in the opposite direction (exo) of the macrocyclic ligand. In the former case, amine protonation and subsequent proton transfer to the metal center are preferred, favoring the formation of a metal hydride, whereas CO 2 attacks directly from the unhindered side in the latter case to generate a manganese hydroxyl−carbonyl complex, the key intermediate for CO production. Hence, the reactivity cannot be rationalized based on the experimentally derived crystal structure due to the fluxional behavior of the carbonyl groups, which to the best of our knowledge has not been reported previously for CO 2 -reducing Mn tricarbonyl complexes. A second factor that affects the mechanism and therefore also the product distribution is the position of the amine functionalities on the macrocyclic ligand. In this respect, ab initio molecular dynamics simulations showed that the distance between Mn and the protonated amines is shortest for the middle amine of complex 1a, lowering the energy barrier for proton transfer to the metal center compared to the case where the side-amine was protonated. Thus, hydride formation can be enhanced by tuning the distance between the metal center and the proton shuttles. Finally, the accessibility of CO 2 as well as protons to the active site should always be kept in consideration, as it is crucial for switching reaction pathways from one to another.
These computational findings align well with the experimental results, where H 2 was the dominant product for all three catalysts (1a, 1b, and 1c), while only small amounts of CO and HCOO − were produced due to disfavored CO 2 addition reactions before and after hydride formation, respectively. Formation of HCOO − was further suppressed with complex 1c, where the linker was shortened by one atom, thus limiting CO 2 insertion and favoring a second protonation after formation of the hydride. The observed product distribution is furthermore in contrast with our previous work, where complex 1a* with a similar but "open" amine ligand produced HCOO − as the dominant product, substantiating the importance of considering the geometric structure when designing new molecular catalysts.